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FDISK(8)		    System Manager's Manual		      FDISK(8)

NAME
       fdisk --	PC slice table maintenance utility

SYNOPSIS
       fdisk [-BIaipqstu] [-b bootcode]	[-1234]	[disk]
       fdisk -f	configfile [-itv] [disk]

PROLOGUE
       In  order  for the BIOS to boot the kernel, certain conventions must be
       adhered to.  Sector 0 of	the disk must contain boot code, a  slice  ta-
       ble,  and a magic number.  BIOS slices can be used to break the disk up
       into several pieces.  The BIOS brings in	 sector	 0  and	 verifies  the
       magic  number.  The sector 0 boot code then searches the	slice table to
       determine which slice is	marked "active".  This boot code  then	brings
       in  the	bootstrap  from	the active slice and, if marked	bootable, runs
       it.  Under DOS, you can have one	or more	slices with one	 active.   The
       DOS  fdisk  utility can be used to divide space on the disk into	slices
       and set one active.

DESCRIPTION
       This command is obsolete.  Users	are advised to use gpart(8) instead.

       The FreeBSD utility, fdisk, serves a similar purpose to the  DOS	 util-
       ity.   The first	form is	used to	display	slice information or to	inter-
       actively	edit the slice table.  The second is used to write a slice ta-
       ble  using  a  configfile,  and	is  designed  to  be  used  by	 other
       scripts/programs.

       Options are:

       -a      Change the active slice only.  Ignored if -f is given.

       -b bootcode
	       Get   the  boot	code  from  the	 file  bootcode.   Default  is
	       /boot/mbr.

       -B      Reinitialize the	boot code contained in sector 0	of  the	 disk.
	       Ignored if -f is	given.

       -f configfile
	       Set  slice  values  using  the file configfile.	The configfile
	       only modifies explicitly	specified slices, unless  -i  is  also
	       given, in which case all	existing slices	are deleted (marked as
	       "unused") before	the configfile is read.	 The configfile	can be
	       `-',  in	which case standard input is read.  See	"CONFIGURATION
	       FILE", below, for file syntax.

	       WARNING:	when -f	is used, you are not asked if you really  want
	       to write	the slices table (as you are in	the interactive	mode).
	       Use with	caution!

       -i      Initialize  sector  0 of	the disk.  Existing slice entries will
	       be cleared (marked as unused) before  editing.	(Compare  with
	       -u.)

       -I      Initialize  sector 0 slice table	for one	FreeBSD	slice covering
	       the entire disk.

       -p      Print a slice table in  fdisk  configuration  file  format  and
	       exit; see "CONFIGURATION	FILE", below.

       -q      Be  quiet.  Benign warnings (such as "GEOM not found") are sup-
	       pressed.

       -s      Print summary information and exit.

       -t      Test mode; do not write slice values.  Generally	used with  the
	       -f option to see	what would be written to the slice table.  Im-
	       plies -v.

       -u      Update  (edit)  the disk's sector 0 slice table.	 Ignored if -f
	       is given.

       -v      Be verbose.  When -f is used, fdisk prints out the slice	 table
	       that is written to the disk.

       -1234   Operate	on  a single slice table entry only.  Ignored if -f is
	       given.

       The final disk name can be provided as a	"bare" disk  name  only,  e.g.
       da0,  or	as a full pathname.  If	omitted, fdisk tries to	figure out the
       default disk device name	from the mounted root device.

       When called with	no arguments, it prints	the sector 0 slice table.   An
       example follows:

	       ******* Working on device /dev/ada0 *******
	       parameters extracted from in-core disklabel are:
	       cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

	       parameters to be	used for BIOS calculations are:
	       cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)

	       Warning:	BIOS sector numbering starts with sector 1
	       Information from	DOS bootblock is:
	       The data	for partition 1	is:
	       sysid 165,(FreeBSD/NetBSD/386BSD)
		  start	495, size 380160 (185 Meg), flag 0
		       beg: cyl	1/ sector 1/ head 0;
		       end: cyl	768/ sector 33/	head 14
	       The data	for partition 2	is:
	       sysid 164,(unknown)
		  start	378180,	size 2475 (1 Meg), flag	0
		       beg: cyl	764/ sector 1/ head 0;
		       end: cyl	768/ sector 33/	head 14
	       The data	for partition 3	is:
	       <UNUSED>
	       The data	for partition 4	is:
	       sysid 99,(ISC UNIX, other System	V/386, GNU HURD	or Mach)
		  start	380656,	size 224234 (109 Meg), flag 80
		       beg: cyl	769/ sector 2/ head 0;
		       end: cyl	197/ sector 33/	head 14

       The  disk  is  divided  into three slices that happen to	fill the disk.
       The second slice	overlaps the end of the	first.	 (Used	for  debugging
       purposes.)

       sysid		     is	used to	label the slice.  FreeBSD reserves the
			     magic number 165 decimal (A5 in hex).

       start and size	     fields  provide  the  start address and size of a
			     slice in sectors.

       flag 80		     specifies that this is the	active slice.

       cyl, sector and head  fields are	used to	specify	the beginning and  end
			     addresses of the slice.

       Note:  these  numbers  are calculated using BIOS's understanding	of the
       disk geometry and saved in the bootblock.

       The -i and -u flags are used to indicate	that the slice data is	to  be
       updated.	  Unless  the -f option	is also	given, fdisk will enter	a con-
       versational mode.  In this mode,	no changes will	be written to disk un-
       less you	explicitly tell	fdisk to.

       The fdisk utility will display each slice and ask whether you  want  to
       edit  it.  If you say yes, fdisk	will step through each field, show you
       the old value, and ask you for a	new one.  When you are done  with  the
       slice,  fdisk  will  display  it	and ask	you whether it is correct.  It
       will then proceed to the	next entry.

       Getting the cyl,	sector,	and head fields	correct	is tricky, so  by  de-
       fault,  they  will  be  calculated for you; you can specify them	if you
       choose to though.

       After all the slices are	processed, you are given the option to	change
       the  "active"  slice.   Finally,	when all the new data for sector 0 has
       been accumulated, you are asked to confirm whether you really  want  to
       rewrite it.

       The  difference	between	 the -u	and -i flags is	that the -u flag edits
       (updates) the existing slice parameters while the -i flag  is  used  to
       "initialize"  them  (old	values will be ignored); if you	edit the first
       slice, -i will also set it up to	use the	whole  disk  for  FreeBSD  and
       make it active.

NOTES
       The  automatic calculation of starting cylinder etc. uses a set of fig-
       ures that represent what	the BIOS thinks	the geometry of	the drive  is.
       These  figures  are  taken  from	 the in-core disklabel by default, but
       fdisk initially gives you an opportunity	to change them.	  This	allows
       you  to	create a bootblock that	can work with drives that use geometry
       translation under the BIOS.

       If you hand craft your disk layout, please make sure that  the  FreeBSD
       slice starts on a cylinder boundary.

       Editing	an  existing  slice will most likely result in the loss	of all
       data in that slice.

       You should run fdisk interactively once or twice	to see how  it	works.
       This  is	completely safe	as long	as you answer the last question	in the
       negative.  There	are subtleties that fdisk detects that are  not	 fully
       explained in this manual	page.

CONFIGURATION FILE
       When  the -f option is given, a disk's slice table can be written using
       values from a configfile.  The syntax of	this file is very simple; each
       line is either a	comment	or a specification, as follows:

       # comment ...
	       Lines beginning with a #	are comments and are ignored.

       g spec1 spec2 spec3
	       Set the BIOS geometry used in slice calculations.   There  must
	       be three	values specified, with a letter	preceding each number:

	       cnum    Set the number of cylinders to num.

	       hnum    Set the number of heads to num.

	       snum    Set the number of sectors/track to num.

	       These  specs  can occur in any order, as	the leading letter de-
	       termines	which value is which; however, all three must be spec-
	       ified.

	       This line must occur before any lines that specify slice	infor-
	       mation.

	       It is an	error if the following is not true:

		     1 <= number of cylinders
		     1 <= number of heads <= 256
		     1 <= number of sectors/track < 64

	       The number of cylinders should be less than or equal  to	 1024,
	       but  this  is not enforced, although a warning will be printed.
	       Note that bootable FreeBSD slices (the "/"  file	 system)  must
	       lie  completely within the first	1024 cylinders;	if this	is not
	       true, booting may fail.	Non-bootable slices do not  have  this
	       restriction.

	       Example	(all  of  these	 are equivalent), for a	disk with 1019
	       cylinders, 39 heads, and	63 sectors:

		     g	     c1019   h39     s63
		     g	     h39     c1019   s63
		     g	     s63     h39     c1019

       p slice type start length
	       Set the slice given by slice (1-4) to type  type,  starting  at
	       sector  start  for  length  sectors.  If	the start or length is
	       suffixed	with a K, M or G, it is	taken as a Kilobyte,  Megabyte
	       or Gigabyte measurement respectively.  If the start is given as
	       "*"  it	is set to the value of the previous partition end.  If
	       the length is given as "*" the partition	end is set to the  end
	       of the disk.

	       Only those slices explicitly mentioned by these lines are modi-
	       fied;  any  slice  not referenced by a p	line will not be modi-
	       fied.  However, if an invalid slice table is present, or	the -i
	       option is specified, all	existing slice entries will be cleared
	       (marked as unused), and these p lines will have to be  used  to
	       explicitly  set	slice information.  If multiple	slices need to
	       be set, multiple	p lines	must be	specified; one for each	slice.

	       These slice lines must occur after any  geometry	 specification
	       lines, if one is	present.

	       The type	is 165 for FreeBSD slices.  Specifying a slice type of
	       zero  is	 the  same as clearing the slice and marking it	as un-
	       used; however, dummy values (such as "0") must still be	speci-
	       fied for	start and length.

	       Note: the start offset will be rounded upwards to a head	bound-
	       ary  if necessary, and the end offset will be rounded downwards
	       to a cylinder boundary if necessary.

	       Example:	to clear slice 4 and mark it as	unused:

		     p	     4	     0	     0	     0

	       Example:	to set slice 1 to a FreeBSD slice, starting at	sector
	       1  for 2503871 sectors (note: these numbers will	be rounded up-
	       wards and downwards to correspond to head and  cylinder	bound-
	       aries):

		     p	     1	     165     1	     2503871

	       Example:	 to set	slices 1, 2 and	4 to FreeBSD slices, the first
	       being 2 Gigabytes, the second being 10 Gigabytes	and the	 forth
	       being the remainder of the disk (again, numbers will be rounded
	       appropriately):

		     p	     1	     165     63	     2G
		     p	     2	     165     *	     10G
		     p	     3	     0	     0	     0
		     p	     4	     165     *	     *

       a slice
	       Make  slice the active slice.  Can occur	anywhere in the	config
	       file, but only one must be present.

	       Example:	to make	slice 1	the active slice:

		     a	     1

FILES
       /boot/mbr  The default boot code.

SEE ALSO
       boot0cfg(8), bsdlabel(8), gpart(8), newfs(8)

HISTORY
       A version of fdisk first	appeared in the	Mach Operating System.	It was
       subsequently ported to 386BSD.

AUTHORS
       fdisk  for  Mach	 Operating  System  was	 written   by	Robert	 Baron
       <rvb@cs.cmu.edu>.    It	 was  ported  to  386BSD  by  Julian  Elischer
       <julian@tfs.com>.

BUGS
       The default boot	code will not necessarily handle all slice types  cor-
       rectly, in particular those introduced since MS-DOS 6.x.

       The entire utility should be made more user-friendly.

       Most  users  new	 to  FreeBSD  do not understand	the difference between
       "slice" and "partition",	causing	difficulty to adjust.

       You cannot use this command to completely dedicate a disk  to  FreeBSD.
       The bsdlabel(8) command must be used for	this.

FreeBSD	13.2			October	5, 2016			      FDISK(8)

NAME | SYNOPSIS | PROLOGUE | DESCRIPTION | NOTES | CONFIGURATION FILE | FILES | SEE ALSO | HISTORY | AUTHORS | BUGS

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